The invention relates to the preparation of serum for clinical analysis. The invention is a device and method used for separating serum from blood by centrifugation, for filtering the separated serum, and for isolating the serum filtrate from the unfiltered portion during storage and decanting.
The invention is a serum preparator used in the preparation of serum for analysis. The serum preparator is a device which filters and partitions serum in conjunction with the centrifugal separation of the cellular and serous components of blood. The serum preparator restrains the initiation and rate of filtration of serum to enable complete centrifugal separation and to optimize the filtration rate. The serum preparator is an isopycnic device which sediments under centrifugation to its equilibrium position within the interface of the separated cellular and serous components and partitions the serum filtrate from the unfiltered components.
Serum is formed when blood is allowed to clot and to separate into its cellular and serous components. The separation of cellular and serous components can be accelerated by centrifugation in a centrifuge tube. During centrifugation, the cellular components sediment to form a cellular pellet and the serum backflows to form a clarified supernatant. The cellular component comprises both clotted blood cells and uncoagulated blood cells. The clot comprises the major cellular component of well clotted blood. The clot sediments quickly. Uncoagulated cells comprise a minor cellular component of well clotted blood. Uncoagulated cells sediment relatively slowly. Separation is complete only after the slowly sedimenting uncoagulated cells have pelleted. Prior art devices similar to the serum preparator have failed to reckon with the slow sedimentation of the minor component of uncoagulated cells. Incomplete separation of the serum results in the contamination of the serum filtrate and compromises the diagnostic utility of the serum analysis.
If the serum is to be processed by an automatic serum analyzer, it should first be filtered to remove fibrin. Fibrin in the serum can cause an automatic serum analyzer to clog. The rate of filtration should be controlled in order to prevent blowby around the filter and to optimize the efficiency of filtration.
To prevent contamination of the serum filtrate during storage and decanting from the tube, the serum filtrate should be partitioned from the cellular pellet. Partitioning is needed to prevent the contamination of the serum filtrate by the diffusion of products from the metabolically active cellular pellet. Such contamination can cause the serum assay to reflect an erroneous level of metabolites, different than the level in the patient's circulation. Also, partitioning is needed to prevent contamination by the cellular pellet during decanting.
Several partition devices are known which are used in conjunction with the centrifugal separation of the components of blood. Such devices are simple barriers which are inserted and travel within a centrifuge tube and which are driven by centrifugal force until they are stopped at the interface by the isopycnic equilibrium of the device. Examples of such devices are disclosed in U.S. Pat. No. 3,508,653 (Coleman) and U.S. Pat. No. 4,001,122 (Griffin). However, these partition devices are unrestrained and sediment simultaneously with the cellular components. These partition devices can overtake and capture the slowly sedimenting uncoagulated cells. This results in the contamination and incomplete separation of the serum.
A partition device which includes means to restrain the initiation of sedimentation is known. The device is disclosed in U.S. Pat. No. 3,972,812 (Gresl). This device comprises a disc of partition material with a substantial void volume. The disc floats atop the samle blood until the void volume fills with fluid, after which the device begins to sediment until it reaches its isopycnic equilibrium. At the isopycnic equilibrium within the interface, the device acts to partition the separated components. The time required for filling the void volume causes sufficient delay for uncoagulated cells to sediment to the cellular pellet ahead of the partition device. However, this device does not teach how to combine a filter device with the partition device.
Devices which combine the filtration and partitioning of serum in conjunction with the centrifugal separation of serum are known. Examples of such devices are disclosed in U.S. Pat. No. 3,931,018 (North) and U.S. Pat. No. 4,202,769 (Greenspan). These devices filter and partition serum in conjunction with the centrifugal separation of the components of blood. However, neither of these devices have means to restrain the initiation of sedimentation or to control the rate of filtration. These devices overtake and capture or lysis the slowly sedimenting uncoagulated cells. Capture causes contamination by incomplete separation. Lysis causes contamination by the lysate. The uncontrolled rate of filtration can cause filter overload and blowby.
A combination filtration and partition device which includes means for restraining the initiation of sedimentation is known. This device is disclosed in U.S. Pat. No. 3,931,010 (Ayres). This device includes a valve which acts as a means for restraining the initiation of sedimentation of the device. The valve in this device is closed at low centrifugation speeds and open at higher speeds. The device sediments only when the valve is open. This enables the separation step to occur prior to the filtration step. The separation step occurs at a low centrifuge speeds with the valve closed and the filtration step occurs at a higher centrifuge speeds with the valve open. However, this device is not an isopycnic device. This device teaches that a mechanical stop is required to stop the sedimentation of such a combination device. Because the position of the mechanical stop is fixed in manufacture, the device is unable to accommodate the separation of blood samples with divergent hematocrits or with divergent sample volumes. The hematocrit of pathological samples of blood can range from 20% to 70%. Serum filtrate from samples with a high hematocrit can be contaminated with cellular material; serum filtrate from samples with a low hematocrit can result in the loss of usable serum. The volume of blood sample to be prepared must be correlated with the position of the mechanical stop. For instance, the preparation of a small volume of blood sample from an infant would require a small sized device. Also, the Ayres device lacks means to restrain the sedimentation velocity of the device and the rate of filtration in order to avoid filter blowby and overload.